The resection of anterior skull base meningiomas has traditionally been performed via pterional or unilateral/bilateral subfrontal craniotomies. The supraorbital keyhole approach and the endoscopic endonasal approach, techniques in which the endoscope is used to aid visualization, were developed to provide alternative, less-invasive approaches to aid the resection of these tumors. The individual characteristics of each tumor, such as location and size, are the main determinants guiding the choice of approach. In this article, the advantages and disadvantages of each approach are discussed, along with complications specific to each technique. Furthermore, a detailed procedural description of each surgical approach is described.
Key points
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The supraorbital keyhole approach and the endoscopic endonasal approach to the anterior skull base allow for minimally invasive resection of anterior cranial fossa meningiomas.
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The supraorbital keyhole approach has been shown to provide higher rates of gross-total resection of anterior cranial fossa meningiomas when compared with endoscopic techniques.
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Rates of postoperative visual deterioration are higher with the supraorbital keyhole approach when compared with endoscopic techniques.
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The endoscopic endonasal approach has higher rates of postoperative cerebrospinal fluid leakage.
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The individual radiographic and anatomic characteristics of each tumor must be the main determinant in choice of surgical approach.
Introduction
Anterior skull base meningiomas comprise a diverse group of tumors that can involve a wide range of locations, including the olfactory groove, planum sphenoidale, tuberculum sellae, parasellar region, anterior clinoid process, and petrous ridge. Collectively, these neoplasms account for approximately 10% of all intracranial meningiomas. Patients typically present with vision loss and headache, although more uncommon symptoms, such as endocrine disturbances, hydrocephalus, anosmia, or extraocular movement palsies, may occasionally be seen.
Historically, the traditional approaches for the resection of anterior skull base meningiomas have focused on several different transcranial routes, including the more common pterional craniotomy, the unilateral subfrontal craniotomy, and the bilateral subfrontal craniotomy. More recently, introduction of the surgical endoscope into the neurosurgical armamentarium has allowed for minimally invasive approaches to the anterior skull base. The supraorbital keyhole approach and the endoscopic endonasal approach, both techniques in which the endoscope aids immensely in intraoperative visualization, have been developed to provide alternative, less-invasive approaches to aid the resection of these tumors. The determination of which approach is most appropriate depends greatly on the anatomic and imaging features of the meningioma in question and its relationship to critical neurovascular structures ( Figs. 1 and 2 ). In this article, we review the supraorbital keyhole and extended endoscopic endonasal approaches for the resection of anterior skull base meningiomas.
Introduction
Anterior skull base meningiomas comprise a diverse group of tumors that can involve a wide range of locations, including the olfactory groove, planum sphenoidale, tuberculum sellae, parasellar region, anterior clinoid process, and petrous ridge. Collectively, these neoplasms account for approximately 10% of all intracranial meningiomas. Patients typically present with vision loss and headache, although more uncommon symptoms, such as endocrine disturbances, hydrocephalus, anosmia, or extraocular movement palsies, may occasionally be seen.
Historically, the traditional approaches for the resection of anterior skull base meningiomas have focused on several different transcranial routes, including the more common pterional craniotomy, the unilateral subfrontal craniotomy, and the bilateral subfrontal craniotomy. More recently, introduction of the surgical endoscope into the neurosurgical armamentarium has allowed for minimally invasive approaches to the anterior skull base. The supraorbital keyhole approach and the endoscopic endonasal approach, both techniques in which the endoscope aids immensely in intraoperative visualization, have been developed to provide alternative, less-invasive approaches to aid the resection of these tumors. The determination of which approach is most appropriate depends greatly on the anatomic and imaging features of the meningioma in question and its relationship to critical neurovascular structures ( Figs. 1 and 2 ). In this article, we review the supraorbital keyhole and extended endoscopic endonasal approaches for the resection of anterior skull base meningiomas.
The supraorbital keyhole approach
The supraorbital keyhole approach is a minimally invasive technique that provides access to a wide range of pathology along the anterior cranial fossa floor and the parasellar region. The addition of endoscopy allows for improved illumination and enhanced visualization of previously hidden areas while still maintaining the benefits of a traditional craniotomy and the familiarity of standard microsurgical dissection techniques. Initial tumor exposure and resection are typically performed with the operative microscope. The endoscope may then be used to magnify the field of view and visualize areas not within the line of sight of the microscope (eg, looking around anatomic corners using angled lenses). Many surgeons advocate for early introduction of the endoscope into the operation to improve visualization of tumor not well visualized using the operative microscope.
The supraorbital keyhole approach is advantageous in its allowance for direct visualization and dissection of tumor from critical neurovascular structures of the anterior skull base. It is truly a minimally invasive approach in that the bone flap typically required is only 3 × 2 cm, and the incision can be made via an eyebrow. It allows the surgeon familiarity of working with standard microneurosurgical instruments during tumor resection, from a standpoint of working between both optic nerves and with an option of working both inferior and superior to the optic chiasm. Additionally, the supraorbital route does not confine the surgeon to working between confined anatomic triangles to reach the suprasellar region, such as the optico-carotid triangle. Meningiomas that extend superior and lateral to the optic nerve, difficult to reach from a transnasal approach, can be easily removed from the supraorbital route. Compared with pterional and subfrontal craniotomies, this approach requires far less brain retraction to visualize the tumor, and does not necessitate splitting of the Sylvian fissure. Also, unlike transnasal approaches, the supraorbital route has a minimal risk of postoperative cerebrospinal fluid (CSF) leakage. Patients who undergo this procedure have the potential for shorter operative times and hospital stays, improved postoperative pain, and better cosmetic outcomes compared with traditional transcranial approaches. With the addition of endoscopy, better operative field illumination and the ability to achieve an angled view of hidden areas are possible.
Drawbacks of the supraorbital approach include the potential for decreased maneuverability through the small craniotomy, the possibility for damage to the frontotemporal branch of the facial nerve, and the risk of entering the frontal sinus during the bony opening. Tumors that extend into the midline depression of the anterior cranial base may prove very difficult to resect without the use of angled endoscopes and instruments. Similarly, difficulty arises in resection of meningiomas that lie below the line of sight along the frontal floor, such as lesion of the sella and lesions with extension below the sphenoid ridge. Finally, there is a relative blind spot behind the ipsilateral optic nerve from which dissection of firm or adherent meningiomas may prove difficult.
The Supraorbital Keyhole Approach Technique
The keyhole approach uses a trajectory along the anterior cranial fossa floor to access a wide spectrum of pathology. Tailoring the head positioning to the pathology involved in each individual case is critical to the success of the surgery. The head should be placed in approximately 20° of extension to enable the frontal lobes to fall away from the anterior cranial fossa floor with gravity. Ipsilateral lesions require 15° to 30° of head rotation toward the contralateral side, whereas more midline and contralateral lesions require 45° to 60° of rotation. Stereotactic navigation guidance is often useful for plotting the approach trajectory and avoiding the frontal sinus. In patients with large frontal sinuses, the surgeon must be prepared to cranialize the sinus in the case of accidental entry or avoid it using a more lateral trajectory.
The supraorbital notch is palpated and marks the medial border of the skin incision. The incision is carried through the eyebrow to the lateral eyebrow margin. In rare cases, it may be necessary to extend the incision past the lateral eyebrow margin in a skin crease. The supraorbital nerve should be preserved to prevent permanent numbness of the forehead. The scalp then can be retracted superiorly and the frontalis muscle cut in the line of the incision. A pericranial flap may be elevated at this point and retracted inferiorly along the supraorbital rim, but is not always necessary if the frontal sinus can be avoided.
The anatomic keyhole is subsequently exposed by incising the temporalis fascia. A small burhole is placed at the keyhole using a 4-mm or 5-mm drill bit, and the dura is dissected off the inner table of the skull to facilitate the craniotomy. A small (3 × 2 cm) craniotomy is often sufficient in providing access to address most pathology. Stereotactic navigation can be used to confirm the frontal sinus will not be entered at the medial border of the craniotomy. Also, the proposed trajectory may be confirmed with the navigation system. In cases with extensive superior tumor extension, the orbital rim can be removed with the frontal craniotomy. The inferior extent of the craniotomy should be as low as possible along the orbital rim to enhance visualization along the anterior cranial fossa floor. The dura is subsequently elevated and any bony protuberances along the anterior cranial base should be flattened with a coarse diamond drill. Additionally, the inner cortex of the frontal bone should be drilled to improve the working angle and aid in visualization.
Attention is subsequently turned to the dura, which is opened and reflected inferiorly ( Fig. 3 ). The operating microscope is brought into the field, and the olfactory tract is readily identified and can be followed posteriorly to the optic nerve and opticocarotid cistern. A retractor can be used to elevate the frontal lobe, but is not always required. The proximal Sylvian fissure, along with the prechiasmatic, opticocarotid, and carotid-oculomotor cisterns, are widely opened with sharp dissection, and cerebrospinal fluid is patiently suctioned out to relax the frontal lobe ( Fig. 4 ). Gravity also will aid in frontal lobe retraction, as arachnoid adhesions are dissected at the base of the frontal lobe and in the Sylvian fissure. The meningioma is identified and its capsule can be opened to permit internal debulking; the tumor is then dissected free from surrounding brain and neurovascular structures using a combination of blunt and sharp dissection.
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